How to Build an AR-15

The AR-15 is a civilian version of the military M-16 main battle rifle. This rifle is destined to be one of the great rifles of all time. The M1 Garand was, in no small part responsible for victory in WWII and as such, was a great rifle. It did not, however, have a long service history. It lasted from it's adoption in 1936 till the early 1950's, where the M14 took over (which was really an M1 variant).

Fairchild (Armalite) sold the rights to the AR-15 to Colt in 1959, when the rifle went into production; and has been ever since. Differing from the M1 in several ways, the M-16 has become a military favorite. While the M1 used a 30-06 cartridge, the M-16 uses the Remington .223 (NATO 5.56mm).

The rifle was designed by Eugene Stoner of Armalite Corp. and was one in a rather long line of military rifles designed by him. Just as with the M1 (and numerous other military rifles), numerous manufacturers have been involved in parts manufacture over the years, including General Motors. There have been numerous attempts to "build a better mousetrap", but thus far, there has been no success.

As a T.I. (the airforce equivalent of the Drill Instructor), I have trained numerous troops on the M-16 rifle. It takes relatively little effort to turn minimal marksmen into expert marksmen. I am always amazed at the comments when watching someone fire the M-16 for the first time. The delight in their eyes tells all; such is the tribute to Eugene Stoner's vision.

The M-16, like all great rifles, is built upon simplicity. The M1, like the M-16 was a simple rifle and enjoyed tremendous success. The simplicity of the rifle is rudimentary to its success. After all, the average soldier is not a rocket scientist and if it's easy to repair, then it will stay functional. This simplicity has a great deal to do with the rifle's success and to a great degree with it's popularity in a civilian version. The fact that it has been the standard military rifle since the 1960's, account for the ready availability of aftermarket parts, books, diagrams and everything else one needs to know to put a rifle together.

Additionally, very few tools are needed to assemble and maintain the M-16/AR-15.

Documents/Videos etc.

If you are planning to do any work on an AR-15, you will need to do your homework. If you buy blind, you WILL be blind-sided and deposit some of that hard earned cash directly in the fecal matter repository; count on it! There are enough junk parts out there, that you had better know what you are buying BEFORE you lay down the cash. There are videos available from Quality Parts and AGI (American Gunsmithing Institute) and both are excellent.

The government publishes several Technical Manuals. They show diagrams, repairs, adjustments, tools and procedures. To no one's surprise, the Army and Marines have differing versions of the same material.There are versions for both the A1 and A2 models.

Here are photos and government numbers for the Technical Manuals:

Army Marine

  • U.S. Army TM 9-1005-319-23&P
  • U.S. Air Force TO 11W3-5-5-42
  • U.S. Marine Corps TM 05538C-23&P/2


These companies can supply a number of reference materials.
Lone Star(800) 482-3733 (512) 681-9280
L.L. Baston(800) 643-1564 (501) 863-5659
Sherwood International(818) 349-7600 (800) 423-5237
Quality Parts (Bushmaster)(207) 892-2005 (800) 998-7928
TAPCO(800) 359-6195
Sierra Supply(303) 259-1822

What To Build

There are several variations of the AR-15. The two greatest variations are the A1 and A2 types. The A1 has a triangular handguard and no elevation adjustment on the rear sight. These are the guns you will see pictures of in books about Viet Nam. The A2 version has a round handguard and an elevation wheel on the rear sight. There are other differences between the two guns such as stock length, forward assist, but these are the differences which can be seen from a distance. The pistol grip is also different between the A1 and A2 versions.

The barrels are available in several lengths and twists and there is also the HBAR - This is a "Heavy Barrel", which means it is less prone to distortion due to heat. The original light barrel is no longer MILSPEC and has not been made for some time. The light barrels are definitely to be avoided. The twists available are 1:7, 1:9 and 1:12. The 1:7 is the current military standard and is designed for the heavier 62 gr. (SS109) bullet. The 1:12 is the original twist and was designed for the 55 grain bullet. The 1:9 twist is a compromise between the two. Be certain that your barrel is made from 4150 steel, NOT 4140! The other barrel option is chrome plating. Accuracy buffs will say that a plated barrel is not as accurate as a regular steel barrel as plating is never uniform. The advantage of chrome plating is a more rugged barrel that is resistant to rust and wear.

See Also Gathering the Parts

Stainless vs. Chrome

Stainless steel is better at preventing erosion than regular 4140 steel, but a 4150 ordnance steel that has been chrome plated will outlast stainless steel. 4150 ordnance steel is the same steel used in military aircraft machineguns and all military small arms barrels. The chrome plated 4150 steel barrel is plated on the inside and gives slightly increased velocity due to the lubricity of the plating.

Barrel lengths available are: 10, 11.5, 14, 16, 20, 24 and 26 Inches. The legal minimum for a rifle barrel is 16 in., so if you buy a barrel shorter than 16 in., it needs to have a long flash suppressor added so it meets the 16 inch minimum required by U.S. law.

What Kind of Barrel is it, anyway?

When contemplating purchasing a barrel from the manufacturer, there is little doubt as to what you are getting, but if you are going to purchase from a "friend" or at a gun show, you cannot always be sure just what you are getting.

Colt marks their barrels: CMP 5.56 NATO 1/9 HBAR

Bushmaster marks their barrels: BMP 5.56 NATO 1/9 HBAR

Armalite (unfortunately) does not mark their barrels, so it is difficult to distinguish between Aramlite and Joe-Bob's barrels.

The MP signifies that the barrel has been magnetic particle checked (magnafluxed), which goes a long way toward identifying flaws in the barrel. Of course, the 1/9 is the twist and may be 1/7 or 1/12.

All of these options are explained in the various catalogs of the companies supplying parts.

Other Options

The standard stock is the nylon rifle stock with a trap door in the butt. The other stock is the Telescoping butt stock, and coupled with a 16 inch barrel makes a nice carbine often referred to as a "shorty" or CAR-15.

There is also an option on the pistol grip. Lone Star Ordinance, Bushmaster and several others sell a pistol grip with a trap door. This grip is available in A1 or A2 styles.

The Lower Receiver

The part that has the serial number on it is the Lower Receiver. This is the only part that cannot be legally purchased without paperwork. To purchase a lower receiver from the manufacturer, you need to be an FFL dealer, Class III manufacturer or hold an NFA Tax Stamp (Form 4). This is a result of the 1986 machine gun ban.

You can make as many barrel/upper receiver assemblies as you like and swap the lower receiver between them, effectively giving you several rifles. The uppers and lowers of all AR-15s will break apart and are interchangeable.

There are a number of manufacturers making lower receivers, however, it is worth noting that the best lowers are made from forgings of 7075-T6 aluminum. This is a very tough and durable alloy and machines very well. There are others that are cheaper and made from cast aluminum. The cast units are definitely inferior in quality, but nevertheless seem to work fairly well. The cast receivers are easy to identify, the numbers, letters and insignia are cast into the receiver; the forged units have stamped numbers, letters and insignia.

Special Tools

You can assemble an AR-15 with a well stocked tool box and some patience. To do the job well with the minimum of frustration, you should either purchase or borrow the special tools. Having someone nearby who has assembled a rifle is always helpful.

Here is a list of tools:

Front Sight Tool
Jaws (GOOD)
Armorers Block
Armorers Block (BETTER)

Barrel Wrench (GOOD)

Barrel Wrench (BETTER)

Rifle kits are available from Quality Parts, Lone Star Ordnance and others. At a minimum, you need to buy the Headspace gauges and the GOOD barrel wrench. Everything else you need, a well stocked workshop will have or will be able to make. The AR-15 is pretty much held together with roll pins. You can put the roll pins in with regular or nipple punches. The nipple punches are better, since they have a small hemispherical protrusion on the business end that rides inside the roll pin, making it far less likely that the punch will slip off, resulting in a marred finish on your fine hardware.

Getting the front detent pin in the lower receiver can be a trial. This is a pin and a spring that holds the front push pin in place. There is a special tool for this, but a 1/4" rod with a .100" hole drilled in one end works well in conjunction with a 1/16" allen wrench or punch. It is one of the trickiest parts of the whole assembly.


Note also that all the springs and pins are not marked, numbered or otherwise identified. You will have to identify them yourself during assembly. There are several that look very similar, so be careful when indentifying parts. For example, the selector lever detent spring looks similar to the take down pin detent spring, but they should not be interchanged. The ejector spring looks similar to the take down detent spring, but should not be interchanged.

Quality Checks on a Completed Rifle

When you have finally assembled the rifle, the next step is to check your work. Remember you are assembling a piece of machinery that could be dangerous if it malfunctions. Go over the rifle and make sure that all parts are correct and tight. Check assemblies with the drawings supplied with the Technical Manuals.

Before you fire the rifle, check the headspace using a set of headspace gauges. The correct use of headspace gauges is explained in the Quality Parts video. If your rifle is not headspaced correctly, contact the company that supplied your barrel and bolt, or take it to a competent gunsmith. If you know how to adjust the headspace, you probably don't need to read this document; but should you be doing so, Quality Parts sells reamers for the AR-15. Note that the headspace check is mostly a safety check, chances are everything will be OK. Later in this document is a complete description of the purpose and process of headspace checking. Military barrels are chrome plated and it is believed that reaming these barrels is not possible because of the risk of flaking chrome. If you have any doubts or questions, call the supplier of your barrel and bolt.

The rifle should be cleaned before firing. The barrel is liable to have a coating of heavy grease inside it. This coating should be aggressively removed with something like Birchwood Caseys Bore Scrubber (not Gun Scrubber), followed by Hoppes number 9 and then a light coating of Break Free CLP.

If all the visual tests pass, then it is a good idea to load a couple of dummy rounds (Snap Caps) into a magazine and manually feed them through the rifle using the charging handle to feed them. Check that the safety works - it will only engage when the rifle is cocked- and that the rounds are ejected when the bolt travels back.

The final test is at a range. Pointing the gun at a solid backstop, load one round in a magazine. Insert the magazine into the rifle and feed it into the chamber. Pull the trigger and make sure that the rifle fires and ejects the shell. With only one round in the chamber, the bolt should be held back by the action of the bolt catch and magazine follower. If this is successful, load two rounds and repeat. The expended case should be ejected and the next round loaded into the chamber. This test ensures that the rifle is feeding and firing properly. If this test passes, insert 5 rounds. Take careful note that each time the trigger is pulled only one round fires. With a new rifle, there may be ejection problems. These can be caused by a rough or sticky chamber, bad lips on the bolt face, a faulty magazine, or a number of other reasons. Check by carefully hand cycling dummy rounds. If the return spring is not strong enough, the rifle will cycle by hand, but will tend to fail to feed in semi-auto use. Try another return spring.

To sight in the rifle and learn how to clean and care for it, you should read the manual: Also check Sighting In

  • U.S. Marine Corps TM 05538C-10/1A
  • U.S. Army TM9-1005-319-10
  • Rifle, M16A2 W/E
  • (1005-01-128-9936)
  • "Operator's Manual W/Components List"

Parts Suppliers

Below is a list of companies that sell complete guns and/or parts for the AR-15.
Armalite(309) 944-6939
Amherst Arms(301) 829-9544
Brownells(515) 623-5401
DPMS(800) 578-3767
Eagle Arms Inc(309) 799-5619
Essential Arms Co(318) 566-2230
Gun Parts (Numrich Arms)(877) 486-7278
L&G Weaponry(714) 840-3772
L.L. Baston(800) 643-1564 (501) 863-5659
Lone Star Ordinance(800) 482-3733 (512) 681-9280
Nesard(708) 381-7629
Olympic Arms (206) 459-7940
Pac West Arms(206) 438-3983
Quality Parts(207) 892-2005 (800) 998-7928
Rock Island(309) 944-5739
Sarco Inc(908) 647-3800
Sherwood International(818) 349-7600 (800) 423-5237
Sherluk Marketing(419) 923-8011

Firing Pins

There is a fad going on about the Titanium firing pins. Titanium is an excellent metal when it comes to strength and durability. (It is one of my favorite metals). However, as it turns out, titanium does not do as well as a good steel when it comes to impact. Save your money, you can buy 6 steel firing pins for the price of a single titanium.

Titanium firing pins are intended to reduce lock time. Theoretically, faster ignition of the shot allows less time for disturbance of the rifle. Since titanium is lighter than the steel normally used in firing pins, it has less inertia, and is therefore accelerated more quickly than a steel firing pin when struck by the hammer. This theoretically results in the firing pin striking the primer faster than a steel firing pin would. Movement of the firing pin in the M16 type rifle is, however, only a very small part of the lock sequence of the rifle. Lightening the firing pin produces virtually no improvement in lock time. Titamium is strong, but doesn't handle impact as well as steel. The steel pin does retain a slight momentum as the bolt carrier closes, and this does cause a slight denting of the primer which can cause a slamfire if the primer is overly sensitive. A titanium pin has less momentum, causes less indent and therefore slightly reduces the possibility of slamfire. In the many thousands of rounds I have fired, I've never had a slam fire; although real, the possibility is remote.


The M16A2 rifle is manufactured in accordance with MILSPEC MIL-R-63997. There are two key elements to the MILSPEC; a verbal description of what the product is and does; and a list of reference documents governing production of the product. In the case of the M16A2 rifle, the key document is Drawing 9349000, which is a package of drawings setting forth the dimensions and tolerance for the rifle.

No commercial, semiautomatic rifle from ANY manufacturer meets both the verbal and technical specifications of the M16A2 rifle. Only M16A2 rifles produced by Colt or FNMI and accepted by the government, fully meet the requirements of the MILSPEC, and they are not legally producible for sale to the public. Even Colt brand commercial rifles are not in conformance with the drawings portion of the MILSPEC.

In a nutshell, if any part you are contemplating purchasing claims full MILSPEC compliance, you may want to rethink that purchase.

Rear Sights

The rear sight of the M16A2 rifle is spring loaded in such a manner that it tends to rotate counterclockwise, as viewed from above. This biasing is caused by a ball and spring in the left wall of the sight base, which presses against a surface of the reciever and forces the rotation. This is a means of taking up accumulated slack in the parts of the sight. The spring loading insures that the sight is always in the same position. Keeping the sights consistently in the same position aids accuracy. Some manufacturers produce rear sights with springs in both sides of the sight. These parts should be avoided, since the sight will tend to lean toward the weaker spring; and as the springs will wear at differing rates, the sights will tend to change position.

Some sights are made so that the rear sight aperture has the flat side facing the shooter. Some are made so that the flat side is away from the shooter. There is no real problem here, it is a matter of taste and opinion. Armalite, for example, feels that the cupped side should face away from the shooter, since the cupped side will tend to reflect light into the shooter's eye.


Headspace is that measurement describing the size of the chamber in a barrel. In the case of rimless rifle cartridges, it is the distance from some arbitrary point on the case neck taper back to the bolt face.


In the more general case (e.g. pistol, rimfire, rimmed cases, and belted magnums), you can imagine headspace as measured from whatever the cartridge rests against on the front of the chamber all the way back to the bolt face.

These numbers are specified for each cartridge by the Sporting Arms and Ammunition Manufacturers Institute (SAAMI) and it's important to have this measurement fall within SAAMI specified tolerances.

Too small, and you may not be able to close the bolt on some ammunition that's on the large side of its tolerance range. Worse yet, it will close and let you fire. Firing ammunition in a chamber that's too tight leads to dangerously high pressures.

Too large a chamber, and there's lots of room for the cartridge to rattle around in there -- well not really. What happens is that the brass casing can rupture and 55,000 psi hot gases start rushing out in every direction, including at your precious body parts. Think of the high pressure gases that force a bullet down the barrel finding other avenues of escape. This is what some people mean by "the gun blew up on me". One sign that this is going to happen soon is that the backs of your brass casings start looking like someone took a sledge hammer to them and flattened out the primers and lettering. This is because the casing is literally being hammered against the bolt face. This can also be caused in reloads by packing the cartridges with too hot of a load.

So that's headspace, and why it's important. But what are all these gauges about? They look like little steel cartridges without the bullets. You stick them into the chamber and try to close the bolt. Whether the bolt closes or not -- and whether that's good or bad -- depends upon which gauge you're trying to use.

Using Headspace Gauges

It is important that you use the headspace gauges properly. Used improperly, the gauges will give false and possibly dangerous information. For example, if you place a NO-GO gauge in the chamber, pull the charging handle all the way back and let it fly, the bolt may well close on the gauge due to the buffer spring pressure. However, done properly, e.g. closing the bolt by hand and trying to turn the bolt, the bolt will not close without excessive hand pressure.

GO GAUGE 1.464"

This gauge is used to determine proper headspace. Insert the "GO" gauge in the chamber, then take the bolt, without the extractor or pin ejector and insert it and turn. The bolt should turn on the gauge with some force. If the bolt does not turn on this gauge, the chamber is not reamed to the correct depth or the bolt is oversized. There is inadequate headspace. Thus, if you can close the bolt, it's a "GO" on the barrel/bolt combination.

NO-GO GAUGE 1.470"

This gauge is also used to determine proper headspace. Insert the "NO-GO" gauge into the chamber, then take the bolt, without the extractor or pin ejector, insert it into the extension and turn. The bolt should not turn on this gauge. Do not force the bolt. If the bolt turns on this gauge, the chamber is too deep or the bolt is undersized. There is excessive headspace. Thus, if you can close the bolt, it's a "NO-GO" on the barrel/bolt combination.


The GO and NO-GO gauges are used to check the limits of factory tolerances between the chamber and bolt to ensure that tolerance build-up is neither inadequate nor excessive. The "FIELD" gauge, checks for headspace in excess of the factory tolerances, thus the gauge is used to determine excessive headspace. Insert the "FIELD" gauge into the chamber, then take the bolt, without the extractor or pin ejector, insert it into the extension and turn. The bolt should not turn on this gauge under any circumstances. If the bolt turns on this gauge, there is excessive headspace and the WEAPON IS VERY DANGEROUS.

Fortunately, since the wear mechanism for a firearm chamber is to stretch, we only have to worry about the dimension beyond the NO-GO measurement; which is our "field" gauge. The field gauge is sized to be the largest acceptable headspace possible. If you can close the bolt on the field gauge, then the barrel is worn out and it's time to replace it. To continue shooting it is dangerous.

In theory, the bolt of a well used rifle will close on a NO-GO gauge too. UNDER NO CIRCUMSTANCES SHOULD A BOLT CLOSE ON A "FIELD" GAUGE. That's when you rebarrel the gun.

Headspace is another matter that needs to be considered when reloading ammunition. In my opinion, it is essential to use a full length resizer die on the cases. This assures that your reloaded ammunition will work in ANY firearm, not just your rifle. The cases stretch and conform to the chamber they were last fired in and if they conformed to a chamber different than yours (and they did) and you don't full length resize, you may be developing inordinately high chamber pressures, which may lead to a very bad day. I also recommend using a factory crimp die in lieu of a standard roll crimp die. This makes reloading life somewhat easier, since you don't have to trim all the cases to the same length, but you still have to trim cases that are over the maximum.

Since you've got a new gun (with less than 10,000 rounds through it), I would suggest you buy both a GO and a NO-GO gauge. Play it conservative, check the headspace every few thousand rounds and don't go beyond the NO-GO. As you know, most people don't even bother with that but I like to play it real safe.

     Basic rules....

    For a new gun:

     a. Closes on the go gauge

     b. doesn't close on the no-go gauge

    For an old rifle that's been rode hard and put up wet:

     a. closes on the go gauge

     b. probably closes on the no-go gauge

     c. definitely does NOT close on the field gauge

It's cheap insurance to have a set around for every rifle you have, especially if you shoot a lot. If you plan to change bolts and/or barrels around, you have to check each combination because each setup could vary a few thousandths. Fortunately you only have to do it when you first introduce them to each other, thereafter, every few thousand rounds if you wish.

Quality PartsHG0223GHG0223NHGO223F

The Front Sight Base

The front sight base can be easily removed after first removing the flash suppressor, handguards, the gas tube pin and the gas tube. If you use a large lead block as a backup support, you will be less likely to marr the finish of the barrel. Notice that the taper pins holding the front sight in place are larger on one end than the other. You will want to drive the pins out from the small side. Some of them are pretty stiff, so just be careful and use just enough force. I recommend a small brass hammer. If you use a larger hammer, you can easily apply too much force and damage something.

Unfortunately, each sight has the taper pin holes drilled in slightly different locations and it is not a simple job to change the front sight base. If you have a new barrel that has not had the taper pin holes drilled, you can make a jig to align the sight with the barrel and then drill the holes. It is critical that the front sight be properly aligned, first with the gas port hole in the barrel and second with the rear sight. I recommend starting the hole with a .125" end mill to approximately half way through. Then change to a .120" drill bit and finish the hole. Then come back with a 5/64" taper pin reamer (available from ENCO). Be careful not to go too deep, otherwise the taper pin will fall through too far. It is best to do the bulk of the reaming on the mill (it doesn't take too much), then finish the ream job by hand to achieve the best fit.

Pre-Ban vs. Post-Ban: What's the Difference?

For the Complete Statute, see The Brady Bill
When the Crime Bill was enacted in September, 1994, there was significant confusion about what would still be permissible. Years later, some of this confusion remains.

One of the provisions of the Crime Bill is that "it shall be unlawful for any person to manufacture, transfer, or possess a semiautomatic assault weapon." 18 U.S.C. sec. 922 (v) (1). However, the code exempts previously-owned "assault weapons" from this prohibition. 18 U.S.C. sec. 922 (v) (2). In the case of a rifle, a "semiautomatic assault weapon" is

A semiautomatic rifle that has an ability to accept a detachable magazine and has at least 2 of--

  • a folding or telescoping stock;
  • a pistol grip that protrudes conspicuously beneath the action of the weapon;
  • a bayonet mount;
  • a flash supressor or threaded barrel designed to accomodate a flash supressor; and
  • a grenade launcher . . .

18 U.S.C. sec. 921 (a) (30) (B). Traditionally, the frame or receiver of a firearm has been legally equivalent to the firearm itself. 18 U.S.C. sec. 921 (a) (3). This has led some people to believe that a pre-ban receiver, that is, one manufactured before 9/13/94, is the same as a pre-ban "assault weapon." Unfortunately, this is not correct.

Since a lower receiver, by itself, does not have any of the five listed features, BATF doesn't consider it an "assault weapon." This is a good thing, since it means that manufacturers may still make the receivers. However, this also means that a pre-ban lower receiver is not an "assault weapon otherwise lawfully possessed under Federal law on [9/13/94]," and therefore not exempted from the ban. 18 U.S.C. 922 (v) (2).

In essence, there are three categories of AR-15 lower receivers: those manufactured before 9/13/94, which were assembled into "assault weapons" on that date; pre-ban lower receivers which were _not_ assembled into "assault weapons" on 9/13/94; and post-ban manufactured lower receivers. The first class can be re-assembled into an "assault weapon," and can be changed (say, from a fixed-stock model to a collapsing stock). The latter two classes may not be assembled into an "assault weapon."